"passionate ardor in pursuit of an objective or course of action," late 14c., from Old French zel (Modern French zèle) and directly from Late Latin zelus "zeal, emulation" (source also of Italian zelo, Spanish celo), a Church word, from Greek zēlos "ardor, eager rivalry, emulation," "a noble passion" [Liddell & Scott], but also "jealousy;" from PIE *ya- "to seek, request, desire." From mid-15c. as "devotion."